Quercetin for browning food surfaces

ABSTRACT

The present invention relates to a food product with a colorless coating on a surface, said coating comprising a quercetin derivative or an either thereof. A further embodiment of the invention relates to a method for coloring a surface of a food product when heated, particularly when heated in a microwave over.

The present invention relates to a food product with a colorless coatingon a surface, said coating comprising a quercetin derivative or an etherthereof. A further embodiment of the invention relates to a method forcoloring a surface of a food product when heated, particularly whenheated in a microwave oven.

The usage of microwave ovens in homes has increased significantly inrecent years and continues to increase. While microwave cooking of foodsaffords a significant time saving over conventional oven cooking, itsuffers from the disadvantage that food products cooked by microwaveenergy lack the desired degree of surface browning, that particularlythose have that have a crust, such as pies, pizzas, bread, dough's etc.have when cooked in a conventional oven.

The most common reaction responsible for surface browning during cookingof products having a dough crust is the well-known Maillard reaction(i.e. non-enzymatic browning). This reaction occurs between naturallyoccurring reducing sugars and compounds containing an amino group, e.g.amino acids, peptides and proteins, and results in the formation ofcolored melanoidins. The rate at which the Maillard reaction proceeds toform such colored pigments increases significantly with temperature andtime. When foods containing a dough crust, such as for example a frozenpizza, a bread or a snack, are heated in a conventional oven, the crustis heated to considerably higher temperatures than the interior of thefood product, with the high surface temperatures being sufficient toachieve the desired browning.

However, in microwave heating the heat energy is released internallywithin the food product so that the surface remains at a relatively eventemperature with the interior. There is a lack of hot, dry airsurrounding the food product during microwave cooking. In addition, thefood is usually cooked for a much shorter time. Consequently, the highsurface temperatures necessary to achieve browning are not reachedwithin the time required to bake the food product. The surface of theproduct remains moist and pale: the desired development of a nice brownsurface color does not appear. The end-product, although well cooked, isoften perceived as under-cooked by the consumer.

A number of compositions have been proposed to create a desirablebrowned surface of a food product when heated by microwave energy. Suchprior microwave browning compositions typically are based on theMaillard reaction to effect browning, and include one or more componentswhich permit the reaction to take place at lower temperatures or whichincrease the reaction rate. Such compositions typically includecarbohydrates such as for example dextrose, maltodextrin andacetaldehyde compounds which result from pyrolysis of some of the sugarcompounds prior to constitution of the browning composition (see U.S.Pat. No. 5,756,140). However, none of these prior compositions have beenentirely satisfactory due to flavor concerns, the limitation ofachievable color variations on a food product, and costs. Further, thepresence of acetaldehydes and potentially still other compounds from thepyrolysis process may be perceived as less natural by consumers.

EPO481249 proposes a method to use an amount of water soluble tea solidsapplied to a food surface to develop a browned surface on the crust ofsuch a food when heated by microwave energy. The shortcoming of theproposed method is that food products treated with such soluble teasolids retain a distinct flavor and taste of black tea. For most productapplications, this is clearly not desired. It is believed that thissignificant flavor impact is due to the fact that a relatively highconcentration of tea solids is needed to be applied to the food surfacein order to be effective for the development of a desired surfacecoloration. A further major inconvenience of the application is that thefood surface remains moist and soft. Hence, this solution does notprovide the consumer with the impression of a well-cooked product with awell-developed crust. Furthermore, such treated products may retaincertain astringency as well as a certain level of caffeine which may notbe desired by consumers, particularly by children.

Currently on the market and commercially used is “Liquid or powderSmoke” (Red Arrow Products Company LLC, Manitowoc, WI, USA). “Liquid orPowder Smoke” overcomes the currently missing solution for fast browningof food surfaces in microwave applications. However, “Liquid Smoke” maynot be well perceived by consumers. It contains aldehydes which have tobe labeled on the packaging of the food products. Currently, the EFSA(European Food Safety Authority) is investigating the safety of “LiquidSmoke” as a food flavoring agent.

Hence, there is a clear need in the art to replace these substances withnatural, safe compositions which can effectively be used on foodproducts for inducing coloration of food surfaces upon heating forexample in a microwave oven. Further, these compositions should beodorless or at least not having a negative impact on the final flavor ofsuch a treated food product.

The object of the present invention is to provide an improved solutionfor coloring surfaces of food products to be heated thereafter, forexample in a microwave oven, and which overcomes at least some of theinconveniences described above.

The object of the present invention is achieved by the subject matter ofthe independent claims. The dependent claims further develop the idea ofthe present invention.

Accordingly, the present invention pertains to a food product with acolorless coating on a surface, said coating comprising a quercetinderivative according to the general formula:

wherein R1 is OH or an O-glucoside; R2, R5 and R6 are OH; R3 and R7 areH; R4 is OH or an O-glucoside; or an ether of said quercetin derivative,the ether bond being formed at one of the hydroxyl groups of thequercetin derivative.

In a second aspect, the invention relates to a method for coloring asurface of a food product when heated, comprising the steps of i)coating the surface or a part thereof with a colorless coatingcomprising a quercetin derivative according to the general formula ofclaim 1 or an ether of said quercetin derivative, and ii) heating saidproduct in order to develop a color of the surface.

The inventors surprisingly found that appealing yellow-brownish colorsdevelop on the surface of a food product during heating, particularlyduring heating in a microwave oven, if such surface has been coated witha composition comprising a quercetin derivative or an ether of saidquercetin derivative prior to the heating step. Such a composition canbe a solution of a quercetin derivative or an ether thereof, or anextract from a natural source, such as from a plant material, comprisingsaid quercetin derivative or ether. When combining such a surfacecoating further with a chemical base such as a sodium bicarbonate orsodium hydroxide solution, the appearance of the yellow-brownish colorcan be even more intensified and give raise to interesting new colorvariations within the brown range of the color spectrum.

This finding can now advantageously be applied to coat un- or prebakedfood products with a transparent and nearly colorless surface coating,which upon baking in for example a microwave oven will develop ayellow-brown color of the food crust. It is of great advantage that theuse of quercetin derivatives and ethers thereof is a natural solutionand that there are no safety concerns to consumers. Furthermore,products coated with a quercetin derivative or ether thereof do not haveand develop any negatively perceived flavors or odors either before orafter heat treatment. A further advantage is that quercetin derivativesor its ethers, with or without the chemical base, can be applied easilyin appropriate concentrations to such food surfaces without leading tomoist and soft surfaces.

Furthermore, the inventors have found that the invention for colorationof a food product surface in a microwave oven works particularly well,if the food product before the heating in the oven is in a frozen stateand/or if the quercetin derivatives or its ethers are applied first andseparately from the chemical base onto said surface. Best results,however, are achieved by applying quercetin derivatives or its ethersfirst in a first coating onto the surface of the frozen food product andthereafter in a second step applying the chemical base to said coatingof the still frozen food product in a second separate layer. It has beenfound by the inventors that the frozen state of the food product as wellas the separate application of the quercetin derivatives or its ethersfrom the chemical base help to further slow down the color reaction atthe food surface before the heating step e.g. during long term storageof the such treated food product. It is thereby possible to make foodproducts with a quasi invisible colorless coating and which can bestored for an extended period of time with maintaining this coatinginvisible, which when heated in a microwave oven develop very nice andappetizing brown surface colors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Browning reaction of a quercetin-3-β-D-glucoside comprisingsurface coating on dough samples before and after heating in a microwaveoven.

FIG. 2: Browning reaction of a quercetin comprising surface coating ondough samples treated with or without different chemical bases beforeand after heating in a microwave oven.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to a food product with a colorlesscoating on a surface, said coating comprising a quercetin derivativeaccording to claim 1 or an ether of said quercetin derivative.

Thereby, a “colorless coating” is understood as a coating on a foodproduct surface which is transparent and without color. Hence, thecolorless coating does not provide an own, proper color to the foodproduct surface. A consumer looking at a food product with such adefined surface coating will not perceive a color coming from thecoating per se.

The product of the invention can be coated on just one or severalsurfaces, if available. Preferably, the surface selected for the coatingis the exterior face or part of the exterior face of the product whichis visible upon presentation of the food product to a consumer.

The food product according of the invention pertains also to suchproducts, wherein the surface is only partly coated with a quercetinderivative or an ether thereof. Partly meaning a part of the entireproduct surface is coated or treated with the quercetin derivative orits ether. This allows inducing a colored surface of only certain partsof a food product, to apply for example certain designs or figures whichonly appear in color after heating or baking of the product. Further,pictures or short texts could be produced on food surfaces in the sameway as well.

In an embodiment, the O-glucoside of either the R1 and/or R4 of thegeneral formula is a monosaccharide or a disaccharide of moietiesselected from glucose, galactose, rhamnose or xylose. The advantage isthat such quercetin derivatives occur naturally and can be found invarious fruits and vegetables. Furthermore, such low molecular weightmolecules are easier for handling and surface coating applications andtend to have a less intrinsic color of their own, which makes theapplication of the current invention more feasible.

Preferably, the quercetin derivative is selected from the groupconsisting of quercetin, izoquercetin, quercitrin, rutin,quercetin-7-O-glucoside and quercetin-3-O-rhamnoside-7-O-glucoside.Those quercetin derivatives are all natural compounds found in variousdifferent fruits and vegetables. They are safe for human consumption andwell characterized in the scientific literature.

The ether of the quercetin derivative is selected from the groupconsisting of azaleatin, rhamnetin, isohramnetin and tamarixetin. Thosecompounds are ethers formed between at least one of the hydroxyl groupsof the quercetin derivative molecule and an alcohol molecule, mostly amethanol. Such quercetin ethers are frequently found in nature and werefor example identified in sage.

In one of the embodiments, the quercetin derivative or the ether of saidquercetin derivative is derived from an extract of a plant material.

Quercetin, its derivatives and ethers naturally occur in many plantmaterials. It is of an advantage that extracts from such plants, forexample from their fruits, leaves or roots, can be used as a naturalsource. Thereby, the said derivatives and ethers could be extracted andpurified from those plant materials. Alternatively, the extractsthemselves or just partly purified from those sources could be used. Forthe latter case, the product would have a much better acceptance withconsumers as they would be considered more ‘natural’. Furthermore,production costs would be significantly lower than if the compoundswould have to be produced synthetically or to be purified tohomogeneity.

Preferably, the extract is from the plant material selected from thegroup consisting of mango fruit, plum, beans, onion, buckwheat, chicoryand pepper, or a combination thereof. Those plants are all rich ineither one of the quercetin derivatives and/or an ether thereof, and theplant materials are well accepted by consumers as well recognized foodproducts themselves. Hence, they are food grade and safe to consume.

The quercetin derivatives and ethers of the invention further maycomprise optionally a binder or thickener as for example pectin,xanthan, agar, dextrin, a gum adhesive agent or another food-gradehydrocolloid, in order to facilitate the technical applicability of theproduct to a food surface.

In a further embodiment, the amount of the quercetin derivative or theether of said quercetin derivative on the surface of said product is inthe range from 0.001-1.0 mg/cm², preferably from 0.005-0.5 mg/cm², morepreferably from 0.01-0.1 mg/cm². These concentrations of the extract onthe surface allow on one hand to provide a practically colorless foodproduct surface coating before the baking or heating step, and on theother hand allow the food surface to develop a sufficiently satisfyingcolor appearance after the heating in for example a microwave oven.

The food product of the present invention is further coated with asolution comprising a chemical base applied to said surface togetherwith or separately of the quercetin derivative or the ether thereof.Thereby, the chemical base can be directly mixed into the solution orextract comprising the quercetin derivative or the ether thereof, andthe pH of the originally acidic extract can be adjusted to a pH valuebetween pH 7 and pH 8.5, for example. Alternatively, the chemical basecan be applied separately to the surface either before or after applyingthe quercetin derivative or its ether, for example by spraying itdirectly onto said surface. As chemical base for example a solution ofsodium bicarbonate such as conventional baking soda, sodium hydroxide orL-arginine can be applied.

The use of a chemical base together with the quercetin derivative or itsether has the advantage of accelerating the development of the desiredcolor reaction. Thereby, the color appearance develops faster and moreintense upon heating of the product surface. Further, using a developersuch as a chemical base allows reducing the amount of quercetinderivative or its ether necessary for reaching the desired food coloringafter the heating step. Hence, the objective to provide an as colorlessfood surface before heating and a well colored surface after heating canbe achieved in this way.

In a preferred embodiment, the surface coating of the food product ofthe invention comprises less than 10⁻⁵ mMol/cm², preferably less than10⁻⁶ mMol/cm², even more preferably less than 10⁻⁷ mMol/cm² ions of atransition metal, particularly of manganese and/or zinc ions. Theadvantage of having no or only a very limited amount of metal ions inthe food product surface coating is to prevent possible off-tastes ofthe food product as well as a loss of quality due to the presence ofsuch metal ions. Metal ions are known to have some off-taste and toenhance oxidation of certain compounds found in foods such as forexample lipids. Hence, the presence of metal ions may lead to a fasterloss of the food product quality as well as to negative organolepticimpacts due to undesired oxidation reactions.

In an alternative preferred embodiment, the food product of theinvention is further coated with an ion of a transition metal, whereinthe amount of the ion of a transition metal on the surface of saidproduct is in the range from 0.00001-1.0 mg/cm², preferably from0.0001-0.1 mg/cm², more preferably from 0.001-0.05 mg/cm².

It has been observed that the presence of transition metal ions togetherwith a quercetin derivative or its ether has a synergistic effect infurther and faster developing the color reaction at a food surface.Hence, in selecting appropriate concentrations of transition metal ionsin combination with the quercetin derivative or its ether, the intensityand speed of the surface color development can be modified and optimizedaccording to individual specific food applications and preferences.

The metal ions are of a transition metal, wherein the transition metalis selected from the group consisting of Fe, Mn, Co, Cr, Zn and Cu, or acombination thereof. Preferably, the transition metal is selected fromthe group consisting of Zn, Fe, Cu and Mn, or a combination thereof.Different metal ions react differently together with quercetinderivatives or its ethers, resulting in slightly but distinct differentcolor appearances within the brownish range of the color spectrum. Thisagain allows adapting not only color intensity but also the color per sefor an individualized use of the invention according to the desiredproduct application.

The food product of the invention is to be heated, and particularly so,the surface of said food product is to be heated. Typically, suchheating can be achieved in a conventional oven or by any other means ofheating a product or its surface such as for example by exposing theproduct to a heating lamp or an infrared heater. Preferably, the productof the invention is heated in a microwave oven.

It is mainly for food products intended to be heated for a short timeonly and at relative lower surface temperatures that the inventionprovides a good solution to surface coloring. Hence, the invention isadvantageously applied on food products intended for being heated in amicrowave oven. For example, food products of the present invention areheated for at least 2 min at 250 Watts or higher, preferably for atleast 4 min at said Watts in a microwave oven. Alternatively, the foodproducts are heated for 1 min and 20 seconds or longer in a microwaveoven at 600 Watts or higher.

The food product according to the invention mainly pertains, but is notlimited, to products selected from the group consisting of dough, bread,cookies, cereals, bakery products, pizzas, snacks, gratins, cookedpasta, lasagne, cheese and rice dishes, and meat.

Preferably, the food product is a frozen food product before beingheated e.g. in a microwave oven. For example, the product is a frozenpizza; a frozen dough or bread product such as a Panini or Hot Pocketproduct; a frozen gratin, pasta, lasagna, cheese or rice dish.

The advantage of the invention for an application to a frozen foodproduct is that the colorless coating is more stable and remains quasiinvisible for a long period of storage, before developing the desiredbrown surface color upon the heating step, e.g. in a microwave oven.

A further aspect of the invention is a method for coloring a surface ofa food product when heated, comprising the steps of i) coating thesurface or a part thereof with a colorless coating comprising aquercetin derivative according to claim 1 or an ether of said quercetinderivative, and ii) heating said product in order to develop a color ofthe surface.

The method of the invention comprises in a further embodiment the stepof applying to said surface a solution comprising a chemical base beforeheating the product. The step of applying the chemical base to thesurface can be combined with applying the quercetin derivative or itsether by for example providing both compositions in a same solution orextract and applying them together to the surface. However, theapplication to the surface can also be provided separately, by forexample first applying the chemical base solution and thereafter thequercetin derivative or ether solution, or vice versa. However, bothhave to be applied to the product surface before the heating step. Toseparate the individual steps as out-lined above has the advantage thatit allows to separate the reactants to better control the coloringreaction. As a chemical base for example a solution of sodiumbicarbonate, sodium hydroxide or L-arginine can be used.

In a still further embodiment, the colorless coating comprises thequercetin derivative or the ether of said quercetin derivative inencapsulated form. A further embodiment is that the chemical base isencapsulated. A still further possibility is that both, the quercetinderivative or its ether as well as the chemical base are encapsulatedseparately.

Encapsulation technology is well known in the art and could be appliedhere to either the quercetin compounds and/or the chemical base.Condition is that the encapsulation releases its enclosed substancesonce heated above a critical temperature. Advantageously, the twocomponents, the quercetin compounds and the chemical base, would notinteract and react with each other while being encapsulated and presentat the same time in the surface coating of a finished food productbefore the heating step. Upon heating, however, the components would bereleased from their encapsulation and could start to react and interactwith each other. This would allow on one hand to improve color stabilityfor increasing storage and distribution time of such coated foodproducts, and on the other hand the perceived effect of surface coloringduring the heating step could be significantly increased.

A further particular embodiment is the method of the invention, furthercomprising the step of applying to the surface of a food product asolution comprising an ion of a transition metal before heating theproduct in order to develop a color of the surface. The ion of thetransition metal may be encapsulated or not.

Advantageously, the method of the invention is used for products whichare intended to be heated in a microwave oven, for example in-home by aconsumer. Upon heating in the microwave oven, the product does thendevelop a brownish color at the surface, typically for a well baked andappetizing product. Such brownish colors depend with the application,the food product type, the concentration and choice of the differentreactants and can result in a variety of different color aspects.

Further advantageously, the method of the invention is for a foodproduct, wherein the food product is in a frozen state before beingheated in order to develop a color of the surface. It has been found bythe inventors that the method of the invention works particularly wellfor frozen food products as any pre-colorization of the treated surfaceof such a frozen product, e.g. during a period of storage, is minimalbefore the heating step in comparison to for example a same treatedsurface of a non-frozen food product.

In a preferred embodiment the heating of the product is in a microwaveoven from 250 to 1400 Watts, preferably from 600 to 1100 Watts.Advantageously, the method of the invention is used for products whichare intended to be heated in a microwave oven, for example in-home by aconsumer. Upon heating in the microwave oven, the product would thendevelop a yellow-brownish color at the surface, typical of a well bakedand appetizing product. Such brownish colors depend with theapplication, food product type, the concentration and choice of thedifferent reactants and can result in a variety of color aspects,reaching into orange, golden-yellow and brown.

Those skilled in the art will understand that they can freely combineall features of the present invention disclosed herein. In particular,features described for the product of the present invention may becombined with the method of the present invention and vice versa.Further, features described for different embodiments of the presentinvention may be combined.

Further advantages and features of the present invention are apparentfrom the figure and examples.

EXAMPLE 1

50 g of pectin (Pectin Classic CU 401, Herbstreith & Fox KG, Germany)was dissolved in 2′000 g of de-mineralized water, heated at 60° C. for 1hour and the pH adjusted with NaOH to pH 4.5. A 0.1 wt % solution ofQuercetin 3-β-D-Glucoside (from Fluka, Switzerland) was prepared bydissolving 10 mg of Quercetin 3-β-D-Glucoside in a final 10 mL of theabove pectin solution. Subsequently, about 0.45 g of the resultingQuercetin solution was brushed onto the cookie raw dough surfacecovering about 33.2 cm² (circle having a diameter of 6.5 cm), whichcorresponds to a concentration of Quercetin 3-β-D-Glucoside of about0.014 mg/cm² at the cookie dough surface. The cookie doughs were thencooked for 1 min 20 sec in a microwave oven (NN-255 Panasonic) at 600Watts.

An additional set of experiments was carried out following the sameprocedure as described above. However, after application of theQuercetin 3-β-D-Glucoside solution to the dough surfaces, about 0.2 g ofa 1M solution of baking soda in water was sprayed onto the same doughsurfaces before cooking in the microwave oven under the same conditionsas above.

The color analysis was carried out using the CIELab* notation. In theInternational Commission on Illumination (CIE), a color is representedby a point in a color space. The coordinates of such a point are: theluminosity L (L=0: black, L=100: white), a* the amount of red and green(a* positive: red, a* negative: green), and b* the amount of yellow andblue (b* positive: yellow, b* negative: blue). The Color analysis wasregistered using a computer controlled digital camera system (DigiEye,Verivide) with a D65 light source.

The results are shown in FIG. 1. The heating step induced a decrease ofthe L* (luminosity) value and an increase of the b* (blue to yellow)value which indicates that the color of the cookie surface turnedtowards the yellow color. This change was even more pronounced whenbaking soda was added.

EXAMPLE 2

About 0.45 g of a 0.1 wt % Quercetin-pectin solution was brushed ontothe surface of a round LEISI pastry dough covering about 44.2 cm²(circle having a diameter of 7.5 cm), which corresponds to aconcentration of Quercetin of about 0.011 mg/cm² at the cookie doughsurface. The dough pastries were then cooked for 1 min 20 sec in amicrowave oven (NN-255 Panasonic) at 600 Watts.

An additional set of experiments was carried out following the sameprocedure as described above. However, after application of theQuercetin-pectin solution to the dough surfaces, about 0.3 g of a a)sodium hydroxide solution 0.1 M, b) sodium hydroxide solution 1 M , c)sodium bicarbonate 1M, d)L-histidine 0.2 M and e) L-arginine 0.44 M wassprayed onto the same dough surfaces before microwaving under the sameconditions as above.

The results are shown in FIG. 2. The heating step induced a decrease ofthe L* value (Luminosity), the overall colour became darker. The colouris not only becoming darker during baking, but the overall colour changedepending on the chemical base used. With the sodium hydroxide aschemical base, the general colour is less yellow and redder, with thesodium bicarbonate the colour goes from transparent to yellow as well aswith the L-arginine. L-arginine has a pKa value of ca. 12.5 and henceacts here as a food-grade chemical base. L-histidine, with a pKa valueof ca. 6.0, only has a limited impact on the surface pH which has beenadjusted in the pectin solution to pH 4.5. Hence, the result of theL-histidine on intensifying the coloration effect of the quercetin isonly small.

1. A food product with a colorless coating on a surface, the coatingcomprising a quercetin derivative according to the general formula:

wherein R1 is OH or an O-glucoside; R2, R5 and R6 are OH; R3 and R7 areH; R4 is OH or an O-glucoside; or an ether of the quercetin derivative,the ether bond being formed at one of the hydroxyl groups of thequercetin derivative.
 2. The product of claim 1, wherein the O-glucosideis a monosaccharide or a disaccharide of moieties selected from thegroup consisting of glucose, galactose, rhamnose and xylose.
 3. Theproduct of claim 1, wherein the quercetin derivative is selected fromthe group consisting of quercetin, izoquercetin, quercitrin, rutin,quercetin-7-O-glucoside and quercetin-3-O-rhamnoside-7-O-glucoside. 4.The product of claim 1, wherein the ether of the quercetin derivative isselected from the group consisting of azaleatin, rhamnetin, isohramnetinand tamarixetin.
 5. The product of claim 1, wherein the quercetinderivative or the ether of the quercetin derivative is derived from anextract of a plant material.
 6. The product of claim 5, wherein theplant material is selected from the group consisting of mango fruit,beans, onion, buckwheat, chicory, pepper, and a combination thereof. 7.The product of claim 1, wherein the amount of the quercetin derivativeor the ether of the quercetin derivative on the surface of the productis from 0.001-1.0 mg/cm².
 8. The product of claim 1, wherein the coatingof the surface of the product comprises a chemical base.
 9. The productof claim 8, wherein the chemical base is selected from the groupconsisting of sodium bicarbonate, sodium hydroxide, L-arginine, and acombination thereof.
 10. The product of claim 1, wherein the product isa frozen food product.
 11. The product of claim 1, wherein the productis selected from the group consisting of dough, bread, cookies, cereals,pizzas, snacks, gratins, cooked pasta, lasagna, cheese and rice dishes.12. A method for coloring a surface of a food product when heated,comprising the steps of: coating the surface or a part thereof with acolorless coating comprising a quercetin derivative according to thegeneral formula:

wherein R1 is OH or an O-glucoside; R2, R5 and R6 are OH; R3 and R7 areH; R4 is OH or an O-glucoside; or an ether of the quercetin derivative,the ether bond being formed at one of the hydroxyl groups of thequercetin derivative or an ether of the quercetin derivative; andheating the product in order to develop a color of the surface.
 13. Themethod of claim 12, comprising the step of applying to the surface asolution comprising a chemical base before heating the product.
 14. Themethod of claim 12, comprising the step of applying to the surface asolution comprising an ion of a transition metal before heating theproduct.
 15. The method of claim 12, wherein the quercetin derivative orthe ether of the quercetin derivative is encapsulated and/or thechemical base is encapsulated and/or the ion of the transition metal isencapsulated.